Oxygen- and magnesium-isotope compositions of calcium-aluminum-rich inclusions from Rumuruti (R) chondrites

Rout SS, Bischoff A, Nagashima K, Krot AN, Huss GR, Keil K

Abstract

We report oxygen- and magnesium-isotope compositions of Ca,Al-rich inclusions (CAIs) from several Rumuruti (R) chondrites measured in situ using a Cameca ims-1280 ion microprobe. On a three-isotope oxygen diagram, delta O-17 vs. delta O-18, compositions of individual minerals in most R CAIs analyzed fall along a slope-1 line. Based on the variations of delta O-17 values (Delta O-17 = delta O-17 - 0.52 x delta O-18) within individual inclusions, the R CAIs are divided into (i) O-16-rich (Delta O-17 similar to -23-26 parts per thousand), (ii) uniformly O-16-depleted (Delta O-17 similar to -2 parts per thousand), and (iii) isotopically heterogeneous (Delta O-17 ranges from -25 parts per thousand to +5 parts per thousand). One of the hibonite-rich CAIs, H030/L, has an intermediate Delta O-17 value of -12 parts per thousand and a highly fractionated composition (delta O-18 similar to +47 parts per thousand). We infer that like most CAIs in other chondrite groups, the R CAIs formed in an O-16-rich gaseous reservoir. The uniformly O-16-depleted and isotopically heterogeneous CAIs subsequently experienced oxygen-isotope exchange during remelting in an O-16-depleted nebular gas, possibly during R chondrite chondrule formation, and/or during fluid-assisted thermal metamorphism on the R chondrite parent asteroid. Three hibonite-bearing CAIs and one spinel-plagioclase-rich inclusion were analyzed for magnesium-isotope compositions. The CAI with the highly fractionated oxygen isotopes, H030/L, shows a resolvable excess of Mg-26 (Mg-26*) corresponding to an initial Al-26/(27) Al ratio of similar to 7 x 10(-7). Three other CAIs show no resolvable excess of Mg-26 (Mg-26*). The absence of Mg-26* in the spinel-plagioclase-rich CAI from a metamorphosed R chondrite NWA 753 (R3.9) could have resulted from metamorphic resetting. Two other hibonite-bearing CAIs occur in the R chondrites (NWA 1476 and NWA 2446), which appear to have experienced only minor degrees of thermal metamorphism. These inclusions could have formed from precursors with lower than canonical Al-26/Al-27 ratio. (c) 2009 Elsevier Ltd. All rights reserved.